JP6700664B2 - Fabrication of malleable slices for correlated atomic resolution tomography analysis - Google Patents

Description

  The present invention relates to a process for forming a sample that can be used for transmission electron microscopy and atom probe microscopy.

  Tomography is a rapidly advancing imaging technique with wide applications in various fields such as medicine, dentistry, biology, environmental sciences, toxicology, mineralogy, electronics, etc. Tomography can be used to obtain various types of information, such as atomic structure and chemical analysis of a sample, such as an X-ray system, transmission electron microscope (TEM), scanning transmission electron microscope (STEM) and/or Alternatively, the process of forming a 3D image of the sample using various tools such as atomic probe microscopy (APM). 3D tomographic data sets are usually obtained by reconstructing a series of 2D projection images of a sample taken at different angles from a photon (eg optical or X-ray) or electron microscope, or in the case of APM. It is obtained by reconstructing a volume from a series of atoms that have undergone field evaporation and have hit a position-sensitive detector.

  TEM and STEM allow the observer to see very small features on the nanometer scale, allowing analysis of the internal structure of the sample. For convenience, references to TEM and STEM are indicated by the term “S/TEM”, and when referring to preparing a sample for S/TEM, it is understood to include preparing the sample for observation on a TEM or STEM. To be done. The sample must be thin enough to allow many of the electrons in the beam to penetrate the sample and exit on the opposite side. Thin S/TEM samples are known as "lamellas" and are typically cut from bulk sample material. Flake typically has a thickness of less than 100 nm, but may have to be significantly thinner than this for some applications. In S/TEM tomography, an electron beam is passed through the slice, and the degree of rotation is gradually increased to form a series of tilted two-dimensional projections across the thin sample. From these 2D projections, a 3D rendering of the original structure can be constructed.

  Atomic probe microscopes (APMs) typically include a sample mount, electrodes and detector. During analysis, the sample is carried by the sample mount and a positive charge is applied to the sample. The sample usually has the form of a pillar with a narrowed needle-shaped tip. The detector is spaced from the sample and is either grounded or negatively charged. The electrode is located between the sample and the detector and is either grounded or negatively charged. Electrical and/or laser pulses are applied to the sample intermittently, which causes the atoms at the tip of the needle to ionize and separate. That is, it "evaporates" from the sample. Ionized atoms, molecules or clusters of atoms pass through the openings in the electrodes and impinge on the surface of the detector, resulting in ions being detected or "counted". Elemental identification of ionized atoms can be determined by measuring the time of flight of the atom between the needle surface and the detector, which varies based on the mass/charge ratio of the ionized atom. The position of the ionized atoms on the surface of the needle can be determined by measuring the collision position of the atoms on the detector. Therefore, evaporating the sample can construct a three-dimensional map of the components of the sample.

  S/TEM provides more desirable structural data and APM provides more desirable compositional data. Different tomographic data from S/TEM or APM used alone hinders optimal material analysis. Correlated S/TEM-APM tomography utilizes both S/TEM and APM data to obtain valuable structural and chemical information from a sample. The quality of the data can vary depending on various aspects of the sample, such as size, shape and density, and composition and spatial distribution of features in the volume under analysis. Current correlated S/TEM-APM tomography typically uses a columnar needle sample that is nominally cylindrical in shape and contains a region of interest (ROI). The quality of each individual S/TEM image in the tomographic series from the pillar sample is higher than the image quality that can be achieved with the slice sample, as a result of, for example, projection effects that obscure the sample thickness and features. Somewhat low. The quality of the data obtained from APM tomography data acquisition experiments is highly dependent on the three-dimensional arrangement of elements over different areas of the sample. In general, APM samples are inhomogeneous with respect to the elements, and in fact the sample has an indistinguishable number and distribution of evaporative electric fields across the field-evaporated portion of the sample, each region of which is the basis for controlling field evaporation. A nominal hemispherical shape must be formed to simultaneously satisfy the equation Ei=kV/ri. In the above equation, Ei is the evaporation electric field of the i-th element on the surface of the APM sample, ri is the radius of the i-th element on the surface of the APM sample, and V is applied to the sample at a specific time of the data acquisition experiment. The voltage, k, is a proportional constant that depends primarily on the geometry of the electrode and sample. If one or more of the field-evaporating elements on the surface of the APM sample cannot meet the requirements of this field-evaporation equation, the sample cannot be controlled (not correlated with voltage or laser pulse). It can volatilize spontaneously in the form, leading to noise in the data or catastrophic destruction of the sample. In addition, it may be difficult to form a pillar sample with invisible or embedded features of the ROI properly positioned within the pillar. Another disadvantage of using S/TEM to correlate APM with pillar samples is that S/TEM is limited because the field of view of the APM data set is limited to about 50% inside the formed pillar sample. There is a tradeoff between data quality and analysis volume within the APM. In addition, even if the APM successfully survives the ROI, there may be significant distortions and noise in the raw data that cannot be properly corrected during the reconstruction or data rendering stages of the analysis. Often. The current problems of APM data acquisition, reconstruction and analysis are described in, for example, Larson et al. )It is described in. Other problems with correlated S/TEM-APM tomography are, for example, holography, differential phase contrast, phase-plate contrast enhancement, grating imaging and analysis. Advanced S/TEM imaging and S/TEM analysis techniques such as are not well suited for columnar samples.

U.S. Patent Application Publication No. 2013/0319849 U.S. Patent Application Publication No. 2013/0248354 International Application Publication No. 2012/103534 US Pat. No. 7,442,924

Larson et al., "Atom probe tomography spatial reconstruction: Status and directions", Current Opinion in Solid State and Materials Science, p.

  Accordingly, it is an object of the present invention to provide an analytical system that improves correlated S/TEM-APM analysis.

  In some embodiments, samples for correlated S/TEM-APM tomography provide good quality data with S/TEM microstructural accuracy and microstructural resolution and APM chemical sensitivity.

  In some embodiments, high resolution S/TEM tilt-series data and APM data with minimal distortion can be consistently produced in a wide range of industrially relevant materials. , Prepare samples to allow site-specific S/TEM-APM analysis. Milling a very thin slice containing the region of interest (ROI), using it for S/TEM analysis, and then reshaping the slice into a needle-shaped sample for APM analysis. The sample is preferably placed on a sample holder compatible with S/TEM and APM. This provides a new morphology of the sample with the ROI located in a slice embedded in a needle-shaped sample for APM.

  Some embodiments further provide a method of forming a needle-shaped sample for the APM portion of a correlated S/TEM-APM tomography. From the bulk material, a thin slice containing the ROI is formed and the slice is coated with a material selected to complement the field evaporation properties of the elemental constituents of the ROI. After this coating step is completed, flakes are formed into needle-shaped samples for analysis with APM.

  Some embodiments further provide a method of performing a correlated S/TEM-APM analysis. A sample containing the ROI is cut out from the bulk of the sample material and formed into thin flakes. The slices are then analyzed using S/TEM to form an image. Optionally, the sliced sample and mount can be washed to remove contaminants that accumulate during the time of S/TEM imaging and subsequent reprocessing of the sample for APM analysis. This slice containing the ROI is then embedded in the selected material and formed into a needle-shaped sample. The needle sample is then analyzed using APM and the resulting data is merged with S/TEM data and correlated with S/TEM data.

  Although the flakes can be used for S/TEM analysis, the samples prepared according to embodiments of the present invention provide improved APM data and on APM without further observation on S/TEM. Can be analyzed with.

  The features and technical advantages of the present invention have been fairly outlined above so that the detailed description of the invention that follows may be more fully understood. The following describes additional features and advantages of the present invention. It should be understood by those skilled in the art that the disclosed ideas and specific embodiments can be easily utilized as a base for modifying or designing other structures to achieve the same objects as the present invention. is there. Furthermore, those of ordinary skill in the art should understand that such equivalent structures do not depart from the spirit and scope of the invention as set forth in the appended claims.

  For a more complete understanding of the present invention and advantages of the present invention, reference is now made to the following description written in connection with the accompanying drawings.

FIG. 5 shows the FIB system positioned in the initial orientation for preparing sample slices for TEM analysis from the substrate. FIG. 7 is a view showing sample flakes arranged in various orientations on a mount. FIG. 7 is a view showing sample flakes arranged in various orientations on a mount. FIG. 7 is a view showing sample flakes arranged in various orientations on a mount. FIG. 6 shows a sample flake with a material coating. FIG. 4 is a view similar to FIG. 3, showing how the region of interest is located within the tip of a needle-shaped sample. FIG. 6 is a stylized view of a needle-shaped sample with a region of interest located within the needle tip. It is a figure which shows the needle-shaped sample by which a region of interest was arrange|positioned so that it might observe in the direction orthogonal to the long axis of a cone. FIG. 6 shows a needle-shaped sample with a region of interest arranged so as to be viewed in a direction along the axis of a cone. 3 is a flow chart showing sample formation and analysis. 3 is a comprehensive flow chart of processing a sample for S/TEM-APM analysis.

  According to some embodiments, a focused ion beam or other method is used to prepare flakes suitable for S/TEM analysis. After observing, optionally on S/TEM, material is deposited on the thin slices to form thicker structures with embedded slices. The thicker structure is then milled to form a needle-like structure for atomic probe microscopy. Thus, according to this general sequence, the region of interest extracted from the bulk sample can be optimized for S/TEM and then separately, the region of interest can be optimized for APM analysis. be able to. The embedded flake sample structure is further improved over conventional conical or needle-shaped structures with regions of interest that have little change in position within the bulk substrate and the surrounding original matrix material. The field evaporation characteristics are shown. Improved data from these two data sources can be more easily correlated and thus improve the accuracy of the reconstructed three-dimensional microstructure and composition of the sample. By correlating the data from S/TEM with the data from APM, precise structural information from S/TEM and precise elemental information from APM can be obtained.

  According to some embodiments, a sample containing a region of interest (ROI) is cut from the bulk of material using standard FIB techniques. An example of cutting a sample from a bulk material is shown in FIG. The sample stage 106 of the instrument is loaded with bulk sample material 108. The stage 106 can provide multiple axes of motion, including translation, rotation and tilt, in such a manner that an optimal orientation of the sample can be achieved at each step of the laminating process. The FIB column 102 is shown oriented to perform initial milling on bulk sample material to make sample slices for S/TEM analysis. In this embodiment, the substrate 108 is oriented such that the top surface of the substrate 108 is at a right angle to the focused ion beam 104 emitted from the FIB column 102. To protect the area of interest and reduce ion milling products, a protective or capping layer 107 is typically deposited over the area of interest, for example by beam-induced deposition of platinum, tungsten or silicon dioxide. As an alternative to, or in addition to, the beam-induced deposition described above, a protective capping layer may be deposited on the surface of the sample prior to loading the sample into the focused ion beam system. Most of the rough ion beam processing performed to make the slice 110 is performed with the substrate 108 and the FIB column 102 in this orientation. This right-angle milling causes the flakes 110 to taper from the bottom to the top due to the focus (ie, conical shape and converging) of the ion beam 104. That is, the thin piece 110 has a thinner top portion than a bottom portion. In this embodiment, lamina 110 remains firmly bonded to substrate 108 at boundary 114. When the thin piece 110 is formed on a substrate having a width or a length of more than several tens of micrometers, the thin piece 110 is taken out from the substrate 108 and thinned to be electron transparent before being used in S/TEM. There must be. In addition, the material removed from the substrate 108 during milling with the vertically oriented ion beam 104 may redeposit on that side of the flake 110, forming an unwanted foreign material layer 112. Similarly, the use of high energy focused ion beams to form flakes on a substrate results in the material of the FIB milled surface and the species used for ion milling, typically gallium, argon or xenon. Here, a thin element layer 111 mixed with each other is formed. The presence of layers 111 and 112 reduces the quality of the S/TEM analysis. Layers 111 and 112 must be removed or polished away before the flakes 110 are used in S/TEM.

  The FIB system can be repositioned in a tilted orientation in order to post-treat the sample slices using over-tilting, polishing and/or undercutting. Overtilting is the process of removing the taper from the flanks of flakes 110 so that the flanks of flakes 110 are substantially parallel. Polishing is a process of removing from the flakes 110 the amorphous layers 111 and 112 that have accumulated on the flakes 110 by previous initial milling. Undercutting is the process of partially or completely separating the flake 110 from the substrate 108 at or near the boundary 114. The sample stage 106 or the FIB column 102 is rotated by an angle 116 about the long axis of the thin piece 110. That is, the sample stage 106 or FIB column 102 is rotated by an angle 116 with respect to the plane defined by the long axis of the slice 110 and a line perpendicular to the top surface of the substrate 108. In other words, the axis perpendicular to the plane of the paper of FIG. 1 located in the cross section of the lamina 110 shown in FIG. 1, preferably the axis perpendicular to the plane of the paper of FIG. 1 located near the center of the cross section of the lamina 110. The sample stage 106 or the FIB column 102 is rotated about the axis.

  As shown in FIGS. 2A, 2B and 2C, the lamina 120 containing the ROI 122 is transferred to a mechanical mount 124 compatible with both S/TEM and APM systems. Transfer the flakes 120 to the mount using a motorized micromanipulator and use the ion beam induced electron beam deposition process or electron beam induced deposition process or mechanical mechanism or adhesive to place the flakes 120 into the mount. It is attached by a known method. The flakes 120 can be mounted on the mount 124 in any orientation. This orientation is chosen so that it can be observed as desired in S/TEM and APT. For example, FIG. 2A shows a lamella 120 with the ROI 122 oriented in a “vertical” orientation on the mount 124, ie, the lamina upper surface 126 is oriented substantially horizontally. Alternatively, as shown in FIG. 2B, the lamina 120 is mounted 124 such that the original upper surface 126 of the lamina 120 is in a “90 degree round” orientation with the ROI 122 rotated to a substantially vertical position. It can also be placed on top. Another option is shown in FIG. 2C, which shows a slice 120 with ROI 122 mounted in an “upside down” orientation relative to the orientation of the slice as it was extracted from the original bulk material. Has been done. Each orientation can be advantageously used to optimize the region of interest 122 for both sample preparation and APM analysis. The flakes 120 are then shaped as thin as possible on the mount by a series of FIB milling patterns. Ideally, the flakes 120 are formed so that the sidewalls 130 are evenly squared and the flakes 120 are formed as thin as possible to ensure that the ROI 122 is not obstructed during analysis using S/TEM. Mold. The finished flakes 120 are very thin and can have a thickness of less than 100 nm. In some embodiments, the flakes 120 can have a thickness of less than 15 nm. The variation in thickness of the observation area of the flakes containing ROI 122 is preferably less than 25%, more preferably less than 10%, and even more preferably less than about 3% over the entire observation area. Lamina 120 can be formed by known conventional techniques including, but not limited to, mechanical focusing and broad beam ion milling, in addition to the focused ion beam milling method described above. Examples of known conventional techniques include U.S. Patent Application Publication No. "Preparation of Lamellae for TEM Viewing," assigned to Fuller et al., assigned to the assignee of the present invention in its entirety by reference. No. 2013/0319849, assigned to the assignee of the present invention, "High Throughput TEM Preparation Processes and Hardware for Thinning a Lateral Conduction of the Inning Vs. U.S. Patent Application Publication No. 2013/0248354, entitled, and International Application Publication, entitled "TEM Sample Preparation," assigned to the assignee of the present invention and incorporated by reference in its entirety in Blackwood et al. No. 2012/103534 A1 is included. The slices are then analyzed in S/TEM by known processes.

  After the flakes 120 have been analyzed by S/TEM, the flakes 120 and mounts 124 can be subjected to a cleaning process. This S/TEM analysis can include, for example, single S/TEM image formation, EELS, EDS, electron holography, differential phase contrast, phase plate contrast enhancement and electron diffraction analysis. Heating, cooling and even environmental exposure can be part of the S/TEM data acquisition method. Similarly, S/TEM tomography using the appropriate S/TEM technique or a combination of the appropriate S/TEM techniques can be used as part of the correlation with the APM data. The cleaning process may be a series of known photon, plasma, gas or liquid cleanings that remove surface organic contaminants such as carbon, or an etchant that selectively removes or reshapes certain materials contained within the ROI. be able to.

  After the S/TEM analysis is completed, the flakes 120 and mounts 124 are attached, eg, SEM with electron beam induced deposition system or ion beam induced deposition system, FIB with ultra low kV column and suitable ion source, PVD, CVD, etc. Place in a suitable attachment system. As shown in FIG. 3, the flakes 120 are uniformly coated with the selected material 132. The coating material 132 is preferably selected to complement the field evaporation properties of the elemental components within the ROI 122. For example, the coating material 132 can have a mass that is different from the mass of the ROI 122 that facilitates separating the coating material 132 data from the sample 120 data in the APM. The coating material 132 is preferably relatively pure, fine-grained, and conformal so that it can fill the voids and voids, and adheres so that a closely-bonded coating is formed on the surface of the flakes. It is preferable that the adhesive layer is a resin, and it is preferable that it can be attached at a relatively low temperature. Examples of suitable coating materials include CVD silicon and PVD silicon, as well as PVD nickel, PVD cobalt and PVD chromium.

  After this coating process, the flakes are returned to the FIB, subjected to a series of sequential annular mills, and molded by known methods into needle-shaped samples as shown in FIGS. 4 and 5. FIG. 4 shows a slice 120 having a hypothetical needle tip shape shown in dashed lines to provide an understanding of the location of ROI 122 within the tip of a needle-shaped sample. In most cases, it is advantageous for the embedded portion of the flakes to be nominally located along the diameter of the cone. FIG. 5 shows a stylized needle-shaped sample 134 with an exaggerated tip 136 to facilitate understanding of the location and placement of ROI 122 within tip 136. However, it should be understood that the tip 136 is more conical in shape. Any suitable FIB mill process can be utilized to form the needle sample. An example of a preferred FIB mill process is US Pat. No. 7,442, assigned to the assignee of the present invention and entitled “Repetitive Circumferential Milling for Sample Preparation” by Giannuzzi et al. , 924., and disclosed.

  6 and 7 show a reconstructed needle-shaped APM data set 137 with the reconstructed ROI 133 oriented differently. FIG. 6 shows a needle-shaped APM data set 137 formed with the reconstructed ROI 133 in the “vertical” orientation discussed above and shown in FIG. 2A. In this vertical orientation, the reconstructed ROI 133 begins well below the top reconstructed capping layer 138 of the data set, reconstructed from the left of the data set to the right of the data set. And extends nominally at right angles to the long axis of the conical data set 137. FIG. 7 shows the reconstructed needle-shaped data set 137 formed with the reconstructed ROI 133 in the “90 degree” orientation discussed above and shown in FIG. 2B. In this orientation, the reconstructed ROI 133 begins at the top of the data set, extends downward toward the bottom of the reconstructed data set 137, and runs parallel to the long axis of the cone. In the actual sample, the ROI structure 122 is embedded within the bulk of the tip 136, and the ROI structure 122 is embedded at a distance of between about 30 nm and 2 micrometers from the apex of the physical tip 136. Preferably. Coating the ROI 122 with the selected material 132 results in a more uniform electric field over a larger area of the needle tip 136 than is achieved when the ROI is formed on the needle tip formed from bulk sample material. .. At the same time, this electric field, which varies with time and position, is mainly limited to the thin strip shape of the embedded flakes of inhomogeneous material. Since the width of the flakes is small compared to the overall diameter of the tip, the complex tip shape formation required by the governing equations described earlier was formed by molding the needle into the "bulk" of the sample, This is achieved by reshaping the material located on the exposed surface of the embedded flakes, rather than by reshaping the material over the surface of the shaped tip in a more traditional manner. Thus, the narrow area of forming the complex tip allows for more controlled field evaporation, reducing field evaporation products such as microfracture and cluster and uncorrelated evaporation events. The "flake-embedded" APM sample is "more flexible" than the needle sample formed in the bulk sample.

  After the FIB milling process is complete, the needle sample 136 is analyzed and reconstructed with APM and a separate software tool is used to visualize the digitized microstructure and merge with S/TEM data or Correlate with S/TEM data.

  FIG. 8 shows a flow chart of the process of forming the needle-shaped sample of the present invention. First, in step 140, ROIs are identified, quarantined, or marked as needed. At step 142, flakes for S/TEM are formed. The slice sample is then analyzed using S/TEM at step 144. The S/TEM can include 3D and ultra high resolution (UHR) S/TEM. After this S/TEM analysis, in step 146, the flake sample is washed, coated and embedded in a coating machine. Then, in step 148, the sample is subjected to a FIB milling process to form a needle-shaped sample. At step 152, the needle-shaped sample is analyzed with the APM, or the sample is repeatedly analyzed with both the APM and the S/TEM 154 by reciprocating the sample between the APM and the S/TEM. Then, in step 155, the data obtained from S/TEM and APM are correlated using separate software tools, and in step 157 the correlated structural and elemental data and images are displayed. FIG. 9 shows the expected changes within each step of the generic workflow shown in FIG.

  While the invention and advantages of the invention have been described in detail, various changes, substitutions and alterations can be made to the specification without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that you can. Furthermore, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the present disclosure, existing or future developments that perform substantially the same function or achieve substantially the same results as the corresponding embodiments described herein. Any process, machine, manufacture, composition of matter, means, method or step performed can be utilized in accordance with the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

102 FIB column 104 Focused ion beam 106 Sample stage 108 Substrate 110 Flake

Claims (16)

A method of analyzing a sample by both S/TEM and atomic probe microscopy, comprising:
Processing a bulk material using a focused ion beam to form flakes having a thickness of less than 200 nm, including a region of interest;
Extracting the flakes from the bulk material using a micromanipulator;
Forming an image of the region of interest using S/TEM (transmission electron microscope or scanning transmission electron microscope);
After forming the image of the region of interest in the S/TEM to form a thicker structure in which the flakes are embedded, material is deposited on the flakes using beam-induced deposition, physical vapor deposition, or chemical vapor deposition. Depositing the material on the face of the flakes on the region of interest;
A thick structure than the said flakes is embedded, by using ion beam milling to form a needle-shaped sample comprising a portion of said material adhering a portion of the flakes containing the region of interest That
Forming an atomic probe microscope image of the region of interest in the needle sample using an atomic probe microscope.   The method of claim 1, wherein forming an image of the region of interest using the atomic probe microscope comprises forming a three-dimensional map of the needle-shaped sample.   Forming an image of the region of interest using the S/TEM includes forming an image of the sample at different angles of the S/TEM with respect to an electron beam, and obtaining a three-dimensional image of the region of interest by tomography. 3. The method of claim 1 or claim 2 including forming.   4. The method according to any one of claims 1 to 3, further comprising combining the information obtained by the S/TEM and the information obtained by the atomic probe microscope on a display. Further comprising placing the slices on a S / TEM compatible mount both the atom probe microscope, the S / TEM includes a scanning transmission electron microscope, to any one of claims 1 4 The method described. The flakes, having a top surface corresponding to the top portion of the bulk material with extracting the flakes, the flakes, on said mount, said upper surface is disposed substantially becomes horizontal orientation to claim 5 The method described. The flakes, having a top surface corresponding to the top portion of the bulk material with extracting the flakes, the flakes, on said mount, said upper surface is disposed substantially become perpendicular orientation to claim 5 The method described. The flakes, having a top surface corresponding to the top portion of the bulk material with extracting the flakes, the flakes, on said mount, said upper surface is disposed in contact with the mount, according to claim 5 the method of. 9. The method of any one of claims 1-8 , wherein depositing the material on the flakes comprises depositing a material having a mass different from the mass of the region of interest. 10. The method of claim 9 , wherein the material deposited on the flakes comprises silicon, nickel, cobalt and/or chromium. The region of interest is located at between 2 micrometers 30nm from the tip of the needle-shaped sample, the method according to any one of claims 1 to 10.   The method of claim 1, wherein depositing the material on the flakes comprises uniformly coating the flakes with a selected material.   The method of claim 1, wherein depositing the material on the flakes comprises depositing silicon by physical vapor deposition (PVD) or chemical vapor deposition (CVD).   The method of claim 1, wherein depositing the material on the flakes comprises depositing nickel, cobalt, or chromium by PVD.   The method of claim 1, further comprising correlating S/TEM structural information from the region of interest with atomic probe microscope elemental information from the region of interest. 16. The associating of S/TEM structural information from the region of interest with atomic probe microscope elemental information from the region of interest comprises reconstructing a three-dimensional microstructure and composition of a sample. the method of.

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